This invention relates to an exposure device for a copying machine wherein a fluorescent lamp and a fiber lens array are used as a light source and image formation means, respectively, and particularly to an exposure value control system therefor.
The fiber lens described herein is a glass or plastic focusing fiber generally known under a trade name "SELFOC", for example, and whose index of refraction varies consecutively in the radial direction with its axis. Imaging functions can be obtained by cutting the focusing fiber into an appropriate length, and it can therefore be utilized as a substitute for an optical system employing conventional lenses and mirrors. As compared with the case where a more conventional optical system employing lenses or the like is used, the entire exposure device formed of a fiber lens can be made very compact since the optical path length between an original support station and an image forming surface on a photosensitive member can be greatly reduced.
In addition, the use of a fluorescent lamp as the light source is very advantageous in view of the amount of heat generated. When an ordinary tungsten-halogen lamp is used, a temperature at a tube wall may be more than approximately 300.degree. C. In the case of a fluorescent lamp, however, the temperature at the tube wall may lie between 60.degree. C. and 80.degree. C. Therefore, the temperature rise therearound is very low for the latter. Fluorescent lamps are used with a fiber lens array because the fiber array is likely to be affected by high temperature, i.e., its color or the like may change, for example.
In FIG. 1a, there is illustrated an exposure device for a copying machine in which a fiber lens array is used in an imaging optical system. An original placed on a contact glass 1 is illuminated by two fluorescent lamps 2 and 3, and the original image to be copied is projected on a photosensitive member 8 through a fiber lens array 6. Since the surface of the photosensitive member 8 is electrostatically charged in advance with a corona charger, an electrostatic latent image corresponding to the image of the original is formed on the surface thereof. The electrostatic latent image is developed, transferred to a record paper and fixed to obtain the desired copy in accordance with a conventional copying process. In addition, reference numerals 4 and 5 show lamp casings, and 7 shows a protection slit for preventing stray light from reaching the photosensitive member 8.
In case where such fiber lens array 6 as described above is used, it is a general tendency that the width W of the area of the origianl exposed at any one time becomes less than approximately one-forth of the comparable width for the case of the conventional lens optical system, so that a mechanical setting of a selected level exposure becomes rather difficult.
It had been known that the selection or control of the exposure level or value can be accomplished by mechanically selecting or adjusting the width of a slit, such as the slit 7, which is disposed in an optical path extending from the original to the photosensitive member, but the demand for high tolerance for the shape of the slit and the attachment thereof is extremely severe because the width W for exposure is relatively narrow. Thus, it is practically difficult to provide such a slit, particularly when the room available for the slit is substantially reduce in a compact fiber lens system. Furthermore a change in the slit width does not always provide a uniform change in the exposure level. In other words, the change of light output as the slit width is enlarged by an increment of 1 mm from zero will not be linear, or causes very small and large portions in its variation ratio, thus making the control of exposure non-uniform.
In addition, it has also been known to those skilled in the art that the control of the exposure value can be performed by rotating a fluorescent lamp of aperture-type which may effectively illuminate the area of narrow exposure width W. As shown in FIG. 1b, the aperture-type fluorescent lamp has an aperture portion 2A or 3A formed in such a manner that the portion corresponding to an aperture angle .alpha. inside the lamp would not be painted with fluorescent material. Therefore, this lamp has a light distribution characteristic represented by dotted lines. A method of adjusting the exposure by turning such aperture-type fluorescent lamps as explained above includes the disadvantages that the mechanism for turning the lamp is complex, contact failure is apt to occur in a pin portion of the lamp because the lamp is rotated, and the lamp itself becomes rather expensive.
From the standpoint of stability in the amount of exposure, light output from a source of light generally vary with a change in the ambient temperature, voltage variation of a power source, degradation of the lamp through aging, and other factors. Although various lamps can be used as a light source for an exposure device, the ambient temperature around a fluorescent lamp largely affects its variation of light output. In the case of a fluorescent lamp, the luminous efficiency of the lamp drops remarkably in a low temperature environment and the light output. immediately after lighting is as low as 30% of the output at steady state. In an exposure device, i.e., an image forming device for a copying machine, if the light output of a light source lamp, namely, the amount of exposure changes, there occurs an unevenness in concentration in each copy or a difference in concentration from copy to copy, and the desired copy quality can not be obtained. As an exposure amount stabilization device, an indirect exposure amount stabilization system is known to those skilled in the art in which the light output of the lamp is stabilized by detecting a lamp voltage, lamp current or power source potential and thereafter feeding it back to a lighting device. This system, however, is effective for only a light souce lamp wherein its electrical characteristics or luminous properties are primarily determined by its electrical characteristics, as in the case of tungsten lamps, and therefore it is not suitable for a fluorescent lamp because the light output of the fluorescent lamp varies largely with its ambient temperature. In other words, a satisfactory effect for stabilization by such systems can not be expected for the fluorescent lamp. To solve this problem, the aforesaid exposure amount stabilization device has already been proposed in which the light output of lamp is stabilized by receiving lamp light by use of light receiving elements and feeding it back to a lighting device.
Admitting that there is a difference of whether the stabilization is effected indirectly by detecting the power source potential, ambient temperature and the like, or directly by sensing the light output, the output of the lamp in the lighting device is regulated by varying its supply voltage in dependence upon the magnitude of the detected values. Accordingly, in this kind of exposure amount stabilization device, it is ordinary practice to cause the lighting device for a light source lamp to have a power supply of a larger capacity than the rated power of the lamp, and to utilize its difference as a margin for absorbing the change of light output caused by various variation factors. In order to absorb the entire variations of light output in a lamp which provides extremely large variations in light output as in the case of a fluorescent lamp, the capacity of the power supply of the lighting device must be made large enough to meet its demand. Such power supply, however, would be of excessive quality for absorption of the variation under ordinary service conditions, would increase the size of the stabilization device itself, raises the production costs thereof, and provides adverse effects on the life expectancy of the lamp.
So far as the fluorescent lamps is concerned, it is unavoidable that the luminous efficiency of the fluorescent lamp, especially at low ambient temperatures, is reduced remarkably and the light output immediately after lighting becomes approximately 30% as compared with that at its steady state, but such large variation as this will occur only when the ambient temperature is less than 10.degree. C. and at the time of lighting or starting, so that occurrence of such condition is infrequent. Thus, it is not economical to prepare a large capacity power supply for the lighting device capable of absorbing all variations, when some may occur infrequently.
If a lighting device having a relatively small power supply is employed and the possibility of occurrence of large variation of light output at the time of starting is neglected a blank period will be produced during which unclear copies may be produced by the copying machine since the variation of light output can not be compensated for by the regulating capacity of the stabilization device during initial operation.
Although it has been known to heat the tube wall of the fluorescent lamp is preliminarily to reduce the variation of light output at the time of lighting or starting, it has such drawbacks that any heating means, which may often exhibit a safety problem, such as a surface heater and the like is required. Such devices complicate construction and elevate the production costs, and the preliminary heating must be maintained even during a wait-to-see period or when the exposure device is not in operation.
From the foregoing, it is preferable to have such counter-measures that the stabilization device is designed so as to accurately cover a given range of .+-.30%, for example, with respect to the variation of light output in the lamp, and that the dealer or manufacturer advices the user to use the exposure device only after it has been placed under its steady state since the device itself is not so designed that it can absorb larger variations exceeding .+-.30%, thereby preventing the excessive quality of the lighting device.